In recent years, the destruction of the ozone layer has been progressing by discharging chemicals containing fluorocarbon or chlorine that have been used in, for example, refrigerators and air conditioners into the atmosphere, resulting in an increase in the amount of ultraviolet light falling onto the ground. Ultraviolet light has a short wavelength and thereby have high light energy and damage, for example, the skin.
Ultraviolet light is classified, depending on the wavelength, into UV-A (315 to 400 nm), UV-B (280 to 315 nm), and UV-C (100 to 280 nm). UV-C having the shortest wavelength in ultraviolet light is significantly absorbed by various materials and hardly reaches the ground. However, UV-B having the second shortest wavelength acts on the epidermal layer of the human skin to accelerate the generation of melanin pigment by pigment cells and thereby causes sunburn, resulting in a risk of canceration of the pigment cells if the degree of the sunburn is severe. UV-A having the longest wavelength oxidizes the melanin pigment generated due to the UV-B to change the color into brown.
Thus, ultraviolet light highly affects the human health and the environment. Furthermore, as described above, the amount of ultraviolet light falling onto the ground is increasing by the destruction of the ozone layer, and consequently detection of the amount of ultraviolet light with, for example, a smartphone or simplified survey meter, in everyday life has been increasingly demanded. Whichever system is used for the detection, a photoelectric conversion element having a high sensitivity to ultraviolet light is necessary.
The basic structure of a light sensor, which is the known optical receiver detecting the amount of ultraviolet light, will be described based on FIG. 12.
As shown in FIG. 12, in a light sensor 100, for example, a first light-receiving device 110 and a second light-receiving device 120 having the same structure are formed, and a filter 140 cutting light having a wavelength in the ultraviolet region is formed only on the first light-receiving device 110. More specifically, in a P-type semiconductor substrate 101, as the first light-receiving device 110 and the second light-receiving device 120, N-type diffusion layers 111 and 121 having deep junction depths and P-type diffusion layers 112 and 122 having junction depths shallower than those of the N-type diffusion layers 111 and 121 are sequentially formed. Furthermore, on the light-receiving devices, an insulating film 132 and a first wiring layer 137 are sequentially formed, and similarly, an insulating film 133 and a second wiring layer 138, an insulating film 134 and a third wiring layer 139, and an insulating film 135 are formed. Furthermore, on the first light-receiving device 110, a filter 140 cutting specific light, for example, light in the ultraviolet region, such as 300 to 400 nm, is formed.
In the case of the diffusion structure of the light sensor 100, light is absorbed by two photodiodes: a photodiode made of a PN junction constituted by the P-type semiconductor substrate 101 and the N-type diffusion layer 111/121 and a photodiode made of a PN junction constituted between the N-type diffusion layer 111/121 and the diffusion layer 112/122. Accordingly, as shown in (b) of FIG. 13, since the photocarriers by the light reached the deep region of the P-type semiconductor substrate 101 consisting of a silicon substrate can be also photoelectrically converted, the second light-receiving device sensitivity is high in the long wavelength region (550 to 1150 nm).
In contrast, the first light-receiving device 110 provided with a filter 140 cutting specific light (e.g., a filter cutting light of 300 to 400 nm) has a spectral sensitivity such as the first light-receiving device sensitivity shown in (a) of FIG. 13.
As shown in (c) of FIG. 13, the output of the sensitivity to ultraviolet is determined by taking a difference between the output of the first light-receiving device 110 and the output of the second light-receiving device 120 shown in (b) of FIG. 13.